Check minitable and/or extensionregistry on collisions in the fasttable.

This offers big speedups when parsing protos with a large number of unknown fields when they also have a significant number of fields in the fasttable, and at least one of them collides with another known field.

Currently lookups for such fields are done twice; a followup change will pass the results of the lookup back to the generic decoder.

PiperOrigin-RevId: 915179906